A cardiac arrhythmia is a condition in which the heart's normal rhythm is disrupted. Certain types of cardiac arrhythmias, including ventricular tachycardia and atrial fibrillation, may be treated by ablation (for example, radiofrequency (RF) ablation, cryoablation, ultrasound ablation, laser ablation, microwave ablation, and the like), either endocardially or epicardially.
Procedures such as pulmonary vein isolation (PVI) and pulmonary vein antrum isolation (PVAI) are commonly used to treat atrial fibrillation. These procedures generally involve the use of a cryogenic device, such as a catheter, which is positioned at the ostium of a pulmonary vein (PV) such that any blood flow exiting the PV into the left atrium (LA) is completely blocked. Once in position, the cryogenic device may be activated for a sufficient duration to create a desired lesion within myocardial tissue at the PV-LA junction, such as a PV ostium or PV antrum. If a cryoballoon is used as the treatment element of the cryogenic device, the balloon is typically inflated using a fluid refrigerant, enabling an entire outer diameter of the balloon to create a circumferential lesion about the ostium and/or antrum of the PV to disrupt aberrant electrical signals exiting the PV. Additionally or alternatively, the cryoballoon may also be used to treat cardiac wall tissue (for example, left atrial wall tissue), in which case the cryoballoon is positioned in contact with the target tissue.
The success of this procedure depends largely on the quality of the lesion(s) created during the procedure, and it therefore would be beneficial to monitor the temperature of the tissue being treated, in part as an indication of lesion quality. There are several ways in which lesion formation may be assessed, either during or after an ablation procedure. In many known devices, the temperature within the cryoballoon is monitored and correlated with the temperature of the tissue being treated. For example, many devices include a thermocouple located within the cryoballoon, which may be located proximate an outlet of a fluid injection lumen. During cryotreatment, liquid refrigerant (such as nitrous oxide) is injected into the inner balloon via the injection lumen. As the refrigerant is injected into the cryoballoon, it expands into the vapor state and causes a temperature decrease. The refrigerant vapor, which has absorbed heat from the cryoballoon-tissue interface, is drawn out of the cryoballoon, back into the shaft of the device and into the console by the console vacuum pump. In a PV isolation procedure, this cooling effect may cause a circumferential lesion about the PV ostium. Although the thermocouple may be used to measure temperature of the dynamic phase transition of nitrous oxide within the cryoballoon due to the heat exchange, the thermocouple does not monitor temperature of the treated tissue.
Another system for temperature measurement is the fluoroptic temperature measurement system. Fluoroptic temperature measurement involves the use of a special thermo-sensitive phosphorescent sensor located at the end of an optical fiber bundle. An elimination light source (LED, xenon lamp, or the like) is used to generate a light that is conveyed through the optical fiber to the thermo-sensitive phosphorescent sensor. Therefore, the phosphorescent sensor emits light over a broad spectrum. The time required for the fluorescence to decay is dependent upon the sensor's temperature. The illumination source is turned off and the generated light from the excited fluorescent sensor is collected. For example, the emitted light from the fluorescent sensor is conveyed back to an optical detector through optical means (for example, a splitter, coupler, or the like) from which the temperature measurement is calculated.
A further temperature monitoring system includes a fiber Bragg grating (FBG) optical sensor, which reflects particular wavelengths of light and transmits all others. FBGs are widely used in civil and aerospace structural health monitoring, and have recently been used for biomedical applications. Despite its prevalent use, the FBG sensor is sensitive to both strain and temperature, and the wavelength shift is indiscernible between the two. Several techniques have been proposed to address this issue, such as using two FBGs in the same environment, encapsulating the FBG in a tapered waveguide, and using a plurality of FBGs inscribed on the same fiber. However, these methods are complicated and are not cost effective solutions.
For cryotreatment procedures using a cryoballoon, the systems mentioned above would require the use of a secondary or additional medical device inserted into the patient. Measurement of the tissue temperature with the secondary device would be possible before and after the cryotreatment procedure (for example, cryoablation), or measurement of the temperature of the border of the lesion may be possible during the cryotreatment procedure. However, in this case, artifacts in the temperature measurement might exist if the sensor is located near the cryoballoon. For example, the heat exchange between the cryoballoon and sensor might create a rapid decay of the fluorescence.
Therefore, it is desirable to provide a cryoablation system and device that allows for the real-time lesion formation assessment by providing real-time temperature feedback during a cryotreatment procedure. Lesion quality also depends on the quality of contact between the treatment device and the target tissue. It is further desirable, therefore, to provide a cryoablation system and device that allows for real-time feedback regarding strain on the balloon as it is brought into contact with tissue, as this may be indicative of tissue contact quality.